|1000 kWh == 1 MWh|
This figure covers only April, May and half of June. I would expect that the number of hours per day of useful generation will increase in July and August, but that this will be very much less in the winter months.
The figure for CO2 "saved" is amazing, but accurate if compared with a coal fired power station. When you burn coal you can only generate about 2 kWh per kg of CO2 produced. I did some calculations a couple of years back and found that I could easily put three or more times this much CO2 into the air each year by driving a car instead of cycling.
I expect that July and August will be better again, before the shorter days of winter have a less positive effect.
The effect of a shadow
|This shadow doesn't look much, but its effect is surprising|
We have 16 solar panels, wired up as two strings each of 8 panels in series. Because the panels are in series, a shadow like this across just one panel is enough to reduce the useful output of the whole string by a half. For that reason, in the morning one string has half the output of the other.
This isn't important for us as it only happens for an hour or two when the panels are least well aligned with the sun and while the output is in any case quite low. Therefore its overall importance is small.
We knew this would happen for a short period at the start of the day, and this is the reason why we didn't choose to fit two extra panels which could have been placed above the various vents on our roof. If we had placed panels there, these would have been in the shade for virtually the whole day, reducing the output of the string of which they were a part at the peak times, when the panels were best aligned with the sun, by more than the extra panels would ever provide a benefit. More panels would have brought us a lower output at a higher cost. If you're installing a system, it's very important to note where shadows are on your roof, especially at your peak hours (generally around mid-day, but later is important if you face South West and earlier is important if you face South East).
How the weather affects the output
I don't suppose many people would be surprised to learn that dark clouds reduce the output of solar panels. This can actually be quite dramatic. While the peak output of our system is 3995 W, and the system comes very close to this at some point on most cool but sunny days, when the weather is really bad, for instance when we had a hailstorm a couple of weeks back, the output drops as low as 200 W for a period.
Another thing which might surprise some people is that the peak output is not achieved on the hottest days. Once solar panels get too hot they actually generate less. On the hottest days that we've experienced, the peak has been only around 3000 W, 1/4 less than the peak on cooler sunny days. However, these were still good days for our system as they were long bright days and therefore the system stayed at a rate of 3000 W for a long time.
I should note that these effects were expected. Before fitting this system, I played around with small scale solar generation for over 20 years and I've seen the same things happen with all the other panels I've used.
It's working very well so far. Only hindsight can tell us if this was really worth doing, but so far it seems like not only is it a good thing for the environment, but also having paid back 2% of its cost in just a two and a half months it's a good financial investment too even though we had no subsidy to buy the system. If you can get a subsidy where you live, or even if you can't, I currently have no reason not to recommend installing a system like this.
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